The present invention relates generally to vehicle control systems, and more particularly to a traction control system that maximizes wheel traction and vehicle stability by limiting engine output based on driven versus undriven wheel speeds.
Traction control refers the process of controlling a vehicle's wheel rotation or spin under certain conditions. When a force is applied to a tire, it produces a frictional force from the interaction of the tire and road surface. Of interest is the longitudinal force on the driven wheels that is used to accelerate the vehicle, and the tire force from torque generated from the engine. A tire cannot produce a frictional force to accelerate the vehicle without any wheel torque, so the frictional force can be expressed as a ratio of the wheel torque to frictional force, often called the coefficient of friction.
Traction control has traditionally been a safety feature in premium high-performance cars, which otherwise need sensitive throttle input to prevent driven wheels from spinning when accelerating, especially in wet, icy or snowy conditions. In recent years, traction control systems have become widely available in non-performance cars, minivans, and light trucks. In race cars, traction control is used as a performance enhancement, allowing maximum traction under acceleration without wheel spin. When accelerating out of a turn, it keeps the tires at optimal slip ratio to maximize speed out of a turn. Traction control can also help a driver to corner more safely. If too much throttle is applied during cornering, the drive wheels will lose traction and slide sideways. This occurs as understeer in front wheel drive vehicles and oversteer in rear wheel drive vehicles. Traction control can prevent this from happening by limiting power to the wheels.
A driven tire has a slip ratio, which is the wheel speed divided by the actual speed of the vehicle. One can also measure the slip ratio as a percentage (e.g., a slip percentage of 10% means that the tire is moving 10% faster than the road surface). However, the slip ratio is not the same as wheel spin. When torque is applied to a tire, the tire distorts and the tire surface tends to ‘creep’ along the road without actually slipping, so a wheel can be moving faster than the road without any wheel spin. In practice, street tires give 1-3% wheel slip (without wheel spin) under moderate acceleration, and drag tires have much more wheel slip, depending on construction.
A tire's coefficient of friction depends on many factors, such as tire construction, road surface, tire loading, temperature, moisture, etc, but it generally increases with increasing wheel slip, up to a point, and then decreases. FIG. 1 illustrates a graph of coefficient of friction vs. wheel slip for a typical tire in dry conditions, with an optimum wheel slip of around 6-7%. In wet conditions, the optimum wheel slip tends is be much lower. For optimal acceleration, if one wishes to maximize the tire coefficient of friction, one needs to keep the wheel slip at the point of tire maximum coefficient of friction. The present invention is a system for accomplishing this objective.
There are various commercial systems that address the traction control in vehicles. Examples of traction control can be found at:    http://www.racelogic.co.uk/index.php/en/other-products/traction-control    http://www.motec.com.au/m800/m800overview/    http://www.aemelectronics.com/engine-management-systems-9/However, each of these existing systems have a drawback in that a) they do not work as an aftermarket product that can operate with an existing engine computer, thus requiring a complete replacement of the existing engine computer at significant expense, or b) are not integrated into the engine computer control logic and so they cannot perform operations integral to the engine computer (for example, retarding ignition to reduce engine output).
The present invention overcomes the shortcomings of these existing systems and provides a robust and efficient traction control system that maximizes acceleration